Why whooping cough vaccine needed before school starts

California's whooping cough epidemic continues to escalate, with state officials reporting 760 more cases in the past two weeks -- a stark reminder of the need to vaccinate youth before school starts.

On Friday, the state Department of Public Health reported that 6,930 cases have been reported in California since Jan. 1.

That's a rate of infection on track to triple the 2,532 cases recorded for all of 2013.

Elementary, middle and high school outbreaks have been reported in counties all over California.

On average, the state sees 80 to 100 cases of whooping cough a month. But this year, some counties by themselves were matching that rate. Santa Clara County, for example, had 87 new cases in July.

The Tdap vaccine for Whooping Cough prepped at the San Jose High Neighborhood Health Clinic, in San Jose, Calif. on July 26, 2011. (LiPo Ching/Mercury News)

The immunity that early childhood vaccination grants against whooping cough, or pertussis, weakens over time. Unlike the measles vaccine, the pertussis vaccine does not provide lifelong immunity.

"A vaccine is required for school entry, whether public or private," said Paul Leung, immunization program manager for Contra Costa County Public Health, which has 274 cases this year. "It's also required for licensed child care and preschool, as well."

Even if vaccinated, children need a "booster" by seventh grade, he added.

"Your protection wears off over time," whether a person had the disease or a vaccine, he said.

The peak age of illness is 15 years. Adolescents who contract the disease can easily transmit it to infants, who are most at risk for death.

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Whooping cough cases are cyclical, peaking every three to five years as the numbers of susceptible persons in the population increases due to waning of immunity following both vaccination and disease. The last epidemic in California occurred in 2010.

But overall incidence of whooping cough has increased since the 1990s. One reason for the increase is the use of acellular pertussis vaccines, which cause fewer reactions than the whole-cell vaccines that preceded them -- but do not protect as long.

The disease itself is caused by the bacterium Bordetella pertussis, which clings to the tiny hairs that line the human respiratory tract. In a healthy person, the hairs keep the passageways clean. But the pertussis microbe releases toxins that paralyze the hairs and surrounding tissues. So sick people can't clear mucus from their lungs and make a "whooping" noise as they try to breathe.

The start of the school year is an ideal time to make sure youngsters are protected, say experts.

"Whooping cough is spread like a cold," said Leung. "If you have people close together, you can get it that way."